The present invention relates to a cooling water passage structure of an outboard motor.
In general, an outboard motor is equipped with an engine vertically disposed on the uppermost portion of the outboard motor in a state mounted to a hull (boat body), for example. That is, a crankshaft is mounted so as to stand upright, and an output (power) of the engine is transmitted to a propeller through a drive shaft which largely extends downward from the engine. When the engine is a four-stroke-cycle engine, an oil pan is disposed below the engine, and oil accumulated in the oil pan is pumped up by an oil pump and is supplied to the engine, thereby lubricating inside the engine.
Further, an engine of the outboard motor is a water-cooled engine and is provided with a water pump for introducing, into the engine, sea water, lake water or river water as cooling water, and the water pump is disposed below the oil pan and is located in the vicinity of the surface of the water. The water pump is driven by the engine through the rotation of the drive shaft.
For example, Japanese Patent Laid-open Publication No. HEI 8-100658 discloses an outboard motor in which a cooling water supply pipe is extending upward from the water pump and is connected to a cooling water passage in an engine. When the water pump is driven through the rotation of a drive shaft for the engine, water discharged from the water pump is supplied through the cooling water supply pipe to the engine to cool the engine.
Further, a water pressure valve is provided in the cooling water passage in the engine. When the cooling water temperature is still low such as immediately after the start of the engine operation, a thermostat provided in the engine operates to stop the flow of the cooling water for facilitating the warm-up of the engine, which increases the pressure of the cooling water. The water pressure valve serves as a relief valve for preventing an excessive pressure rise by discharging a portion of the cooling water so as to protect a sealing portion of the cooling water passage.
In the conventional outboard motor, the water pressure valve is provided in the cooling water passage in the engine (see Japanese Patent Application Laid-open No. HEI 8-100658).
However, because the water pressure valve is provided in the engine itself, layouts of other parts provided in the engine are limited. Further, it is necessary to provide an exclusive passage or pipe member for flowing cooling drainage discharged from the water pressure valve, which causes a problem that a space in the engine room is reduced, and the structure is made complicated.
Furthermore, since there is no special cooling means for the oil accumulated in the oil pan, there is another problem that the temperature of oil is prone to be increased when the outboard motor is driven at high load.